Hearing impaired individuals often experience at least two distinct problems:
1) A hearing loss, which is an increase in hearing threshold level, and
2) A loss of ability to understand speech in noise in comparison with normal hearing individuals. For most hearing impaired patients, the performance in speech-in-noise intelligibility tests is worse than for normal hearing people, even when the audibility of the incoming sounds is restored by amplification. Speech reception threshold (SRT) is a performance measure for the loss of ability to understand speech, and is defined as the signal-to-noise ratio required in a presented signal to achieve 50 percent correct word recognition in a hearing in noise test.
In order to compensate for hearing loss, today's digital hearing aids typically use multi-channel amplification and compression signal processing to restore audibility of sound for a hearing impaired individual. In this way, the patient's hearing ability is improved by making previously inaudible speech cues audible.
However, loss of ability to understand speech in noise, including speech in an environment with multiple speakers, remains a significant problem of most hearing aid users.
One tool available to a hearing aid user in order to increase the signal to noise ratio of speech originating from a specific speaker, is to equip the speaker in question with a microphone, often referred to as a spouse microphone, that picks up speech from the speaker in question with a high signal to noise ratio due to its proximity to the speaker. The spouse microphone converts the speech into a corresponding audio signal with a high signal to noise ratio and transmits the signal, preferably wirelessly, to the hearing aid for hearing loss compensation. In this way, a speech signal is provided to the user with a signal to noise ratio well above the SRT of the user in question.
Another way of increasing the signal to noise ratio of speech from a speaker that a hearing aid user desires to listen to, such as a speaker addressing a number of people in a public place, e.g. in a church, an auditorium, a theatre, a cinema, etc., or through a public address systems, such as in a railway station, an airport, a shopping mall, etc., is to use a telecoil to magnetically pick up audio signals generated, e.g., by telephones, FM systems (with neck loops), and induction loop systems (also called “hearing loops”). In this way, sound may be transmitted to hearing aids with a high signal to noise ratio well above the SRT of the hearing aid users.
In all of the above-mentioned examples a monaural audio signal is transmitted to the hearing aid.
However, in a situation in which a user of a conventional binaural hearing aid system desires to listen to more than one of the above-mentioned audio signal sources simultaneously, the user will find it difficult to separate one signal source from another.
U.S. Pat. No. 8,208,642 B2 discloses a method and an apparatus for a binaural hearing aid in which sound from a single monaural signal source is presented to both ears of a user wearing the binaural hearing aid in order to obtain benefits of binaural hearing when listening to the monaural signal source. The sound presented to one ear is phase shifted relative to the sound presented to the other ear, and additionally, the sound presented to one ear may be set to a different level relative to the sound presented to the other ear. In this way, lateralization and volume of the monaural signal are controlled. For example, a telephone signal may be presented to both ears in order to benefit from binaural reception of a telephone call, e.g. by relaying of the caller's voice to the ear without the telephone against it, albeit at the proper phase and level to properly lateralize the sound of the caller's voice.
Hearing aids typically reproduce sound in such a way that the user perceives sound sources to be localized inside the head. The sound is said to be internalized rather than being externalized. A common complaint for hearing aid users when referring to the “hearing speech in noise problem” is that it is very hard to follow anything that is being said even though the signal to noise ratio (SNR) should be sufficient to provide the required speech intelligibility. A significant contributor to this fact is that the hearing aid reproduces an internalized sound field. This adds to the cognitive loading of the hearing aid user and may result in listening fatigue and ultimately that the user removes the hearing aid(s).